1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device capable of gradation display.
2. Description of the Background Art
Conventionally, liquid crystal display devices for displaying a static or dynamic image have been utilized in personal computers, television receivers, portable telephones, personal digital assistants, and so forth.
FIG. 4 is a circuit diagram showing main parts of such a liquid crystal display device. In FIG. 4, the liquid crystal display includes a liquid crystal cell 30, a vertical scanning line 31, a common interconnection line 32, a horizontal scanning line 33 and a liquid crystal driving circuit 34, the liquid crystal driving circuit 34 including an N channel MOS transistor 35 and a capacitor 36.
N channel MOS transistor 35 is connected between horizontal scanning line 33 and one electrode 30a of liquid crystal cell 30, the gate thereof being connected to vertical scanning line 31. Capacitor 36 is connected between electrode 30a of liquid crystal cell 30 and common interconnection line 32. A power-supply potential VCC is applied to the other electrode of liquid crystal cell 30, and a reference potential VSS is applied to common interconnection line 32. Vertical scanning line 31 is driven by a vertical scanning circuit (not shown) and horizontal scanning line 33 is driven by a horizontal scanning circuit (not shown).
When vertical scanning line 31 is set to a level xe2x80x9cH,xe2x80x9d N channel MOS transistor 35 is conducting, and electrode 30a of liquid crystal cell 30 is charged to the level of horizontal scanning line 33 via N channel MOS transistor 35. For example, the light transmittance of liquid crystal cell 30 will be minimum when electrode 30a is at the level xe2x80x9cH,xe2x80x9d while the light transmittance of liquid crystal cell 30 will be maximum when electrode 30a is at a level xe2x80x9cL.xe2x80x9d A plurality of such liquid crystal cells 30 are arranged in a plurality of rows and columns to form a liquid crystal panel, the panel displaying an image.
A conventional liquid crystal display device has been configured as described above, so that, in order to perform gradation display in one liquid crystal cell 30, an application of an analog potential corresponding to the gradation was required.
However, when an image is displayed in response to a digital image signal, a digital-to-analog conversion circuit will be required for converting a digital signal to an analog signal, leading to a problem of higher cost.
A main object of the present invention is, therefore, to provide an inexpensive liquid crystal display device capable of gradation display.
A liquid crystal display device according to the present invention includes a liquid crystal cell receiving a power-supply potential at one electrode thereof and having a light transmittance varied in accordance with a potential applied to the other electrode thereof, a variable capacitance circuit connected between a line of a first reference potential and the other electrode of the liquid crystal cell and having a capacitance value controllable in a plurality of steps, and a control circuit selectively setting the capacitance value of the variable capacitance circuit in response to an image signal to set a potential of the other electrode of the liquid crystal cell. Thus, the light transmittance of the liquid crystal cell can be varied by changing the capacitance value of the variable capacitance circuit, so that gradation display can be performed with one liquid crystal cell without adding a digital-to-analog conversion circuit, and hence the cost of the device will be reduced.
Preferably, the variable capacitance circuit includes a plurality of first capacitors each having one electrode connected to the other electrode of the liquid crystal cell, and a plurality of first switching elements connected, each at one electrode, to the other electrodes of the plurality of first capacitors, and receiving, each at the other electrode, the first reference voltage. The control circuit renders conductive or non-conductive each of the plurality of first switching elements to selectively set the capacitance value of the variable capacitance circuit. In this case, the light transmittance of the liquid crystal cell can be changed by the number of the first switching elements to be conducted.
Further, each of the plurality of the first capacitors preferably has a capacitance value different from each other. In this case, gradation display in a larger number of steps will be possible.
It is also preferable to provide a second capacitor having one electrode connected to the other electrode of the liquid crystal cell, and receiving, at the other electrode, a second reference potential. In this case, more accurate setting of the potential of the other electrode of the liquid crystal cell will be possible.
More preferably, a plurality of second switching elements connected, each at one electrode, to the other electrodes of the plurality of first capacitors and receiving, each at the other electrode, the second reference potential, and a third switching element having one electrode connected to the other electrode of the liquid crystal cell and receiving, at the other electrode, the second reference potential, are further provided. The control circuit renders conductive the plurality of second switching elements and the third switching element before setting a potential of the other electrode of the liquid crystal cell, to reset the potential of the other electrodes of the plurality of first capacitors and the other electrode of the liquid crystal cell to the second reference potential. In this case, residual charge in the first capacitors and the liquid crystal cell can be removed, so that the potential of the other electrode of the liquid crystal cell can more accurately be set.
It is also preferable to further provide a fourth switching element having one electrode connected to the other electrode of the plurality of first switching elements, and receiving, at the other electrode, the first reference potential. The control circuit renders non-conductive the fourth switching element after setting a potential of the other electrode of the liquid crystal cell to stop feeding of the first reference potential to the other electrodes of the plurality of first switching elements. This can prevent variation of the other electrode of the liquid crystal cell due to leakage current of the first switching elements.
More preferably, each of the plurality of first switching elements is a field effect transistor, and a plurality of third capacitors connected, each at one electrode, to respective input electrodes of the plurality of the field effect transistors and receiving, each at the other electrode, a third reference potential, is further provided. The control circuit charges or discharges one electrode of each of the plurality of third capacitors to renders conductive each of the plurality of field effect transistors.
Further, the liquid crystal display device is preferably installed in a portable electronic device. The present invention will be particularly advantageous in such a case.